How many moles of o2 are required for the complete reaction of 54.7 g of c2h4 to form co2 and h2o?

The balanced chemical equation that illustrates this reaction is:
C2H4 + 3O2 --> 2CO2 + 2H2O 

From the periodic table:
mass of carbon = 12 grams
mass of hydrogen = 1 gram
Therefore:
molar mass of C2H4 = 12(2) + 4(1) = 24 + 4 = 28 grams

number of moles = mass / molar mass
number of moles of C2H4 = 54.7 / 28 = 1.95 moles

From the balanced equation above:
3 moles of oxygen are required to react with one mole of C2H4, therefore, to know the number of moles required to react with 1.95 moles of C2H4, all you have to do is cross multiplication as follows:
number of oxygen moles = (1.95*3) / 1 = 5.85 moles

About 8 gallons of fuel will be used. (8.08)